Applications of alloy steels

Alloy steels may be divided into four classes:
(1) Structural steels, which are subjected to stresses in machine parts.
(2) Tool and die steels.
(3) Magnetic alloys.
(4) Stainless and heat-resisting steels.

Structural steels

The structural steels can be grouped conveniently on the basis of
tensile strength. However, the dividing lines between the classes
are ill defined owing to the wide variation in properties obtained
from one steel by varying the heat-treatment, and the ruling section
in which the properties are required.

The basis of design of machine components generally fall
into either static or dynamic loading. For static loading the proof
stress should be used with a safety factor of 2 or less. For dynamic
loading the working stresses must be related to the fatigue limit,
which is about 55% of tensile strength for mild steel, but only 50%
at 770 MPa and 40% at 1540 MPa tensile strength. Notch sensitivity
is also very important.

Structural Steels Below 680 MPa
tensile

The main objective of steels in this class is to enable
lighter structures to be built by the use of relatively high tensile
steels, while retaining as far as possible the highly desirable
properties. Properties include easy workability, adaptability and
insensitivity to faulty manipulation possessed by mild steel. These
steels may be used in the hot rolled condition.

From the point of view of cost, therefore, quench and tempering
processes on thick plates and sections are usually ruled out.
However, the installation of special equipment, such as roller
quench presses, may make the technique an alternative to expensive
alloy additions.

Further, although killed steels are established in European
technology, balanced steels remain in favor in UK because hot top
practice is not widely used and a greater yield is obtained in
balanced steels. Increased use of continuous casting may affect this
in the future.

The most popular types are those with manganese raised to
1.3-1.7% with carbon 0,2-0,4%, but for welding the carbon is kept
low. The American "Corten A" Steel has a composition of C, 0.12; Si,
0.5; Cu, 0.5; Cr, 0.8; P, 0.1 and Mn, 0.5%. Although the tensile
strength is less than 494 MPa the yield is in the region of 371
MPa.

The combination of copper and phosphorus also increases the
resistance to atmospheric corrosion which is important when thinner
plates are used. The original steel "A" suffers a decrease in yield
strength and notch ductility in thickness over 25 mm, to overcome
which "Corten B" was developed-C 0.14; P 0.04; Mn 1.1; Cr 0.5; Cu
0.4; V 0.1; Bol Al 0.02.

The addition of 0.5 nickel and 0,25 % molybdenum to a manganese
steel gives a good general purpose steel (785 M 19). Fortiweld steel
containing Mo, 0.5; B, 0.003; C, 0.11%. has a TS of 618 MPa and is
readily welded since it transforms in the bainitic region.

Pearlite Reduced Steel. Pearlite increases the tensile
strength but not the yield stress, and since it raises the
brittle-ductility transition temperature, there is a good case for
reducing the carbon content. Low carbon steels (< 0.15%)
strengthened with Mn + Nb and control rolled have good weldability
and toughness and are called Pearlite Reduced Steel (PRS).

Grain refinement. Decreasing the ferrite grain size
significantly increases both yield strength and notch ductility
without increasing the carbon equivalent, which affects weldability.
The relation of yield strength sy to structure is given by the Petch
equation:

sy =
si + Ky
d-1/2

Fine grained steels using Al N have to be produced as killed
steel with low (production) yield (BS 50D). Niobium and vanadium
have a lower affinity for oxygen than aluminium and can be used in
semi-killed steels, an economic advantage. Since 1960 about
0.30-0.1% Nb forming Nb3C4 has been increasingly used as a grain
refiner and precipitation hardening element and is the basis of
several weldable steels in BS 4360, replacing 968 7762 3706, and
includes 4 tensile ranged (40, 43, 50, 55 h bar) with several
sub-grades which are distinguished by increased stringency of yield
stress and notch ductility requirements. Both ladle and product
analyses and carbon equivalents are included.

By controlled hot rolling from 1250°C to a low finishing
temperature (900°C) with a substantial amount of deformation in
range 950-850°C, a fine ferrite grain size is obtained in sections
up to 25 mm, with a minimum yield of 463 MPa due to dispersion
hardening occurring in the ferrite during cooling.

Thick plates present difficulties in getting the required drop in
rolling temperature. Holding at an intermediate temperature produced
a partially recrystallised structure of large grains surrounded by
small ones. The final structure is of very mixed size with poorer
mechanical properties.

It is possible to quench similar steels from 1050°C to form a low
carbon martensite or with lower carbon content, acicular ferrite
followed by tempering to give higher properties. Nicuage steel, C,
0.06, Ni, 1, Cu, 1.1, Nb, 0.02, Mn, 0.5 rolled and aged at 500-570°C
has a yield of 600 MPa and a tensile strength of 700 MPa, elongation
23% and 34-82 Joules charpy V notch impact at -20°C coupled with
good weldability and corrosion resistance.

Steels Above 680 MPa Tensile

Relative to the steels just discussed, those in this group are
designed solely for their mechanical properties, which depend on
accurately controlled heat-treatment.

In 1941 BS 970 covering bars, billets and forging in this
strength range rationalized steel specifications to conserve
essential alloys. The basic principle was the specifying of
mechanical properties related to size of bar when heat-treated
rather than chemical composition. The cheapest steel will develop
the requisite properties in the limited ruling section of the
component used although other factors may modify the choice such as
forging characteristics and die wear, machinability and ease of heat
treatment.

Suitable compositions within the range have to be chosen in
relation to mass. Thus nickel alone has a low efficiency in spite of
its pre-war popularity. A conception new to many engineers is that
the actual steel used for a given tensile strength depends on
the size of the article at the time it is heat-treated.

In 1970, BS 970 was revised and the En designation was replaced
by a six digit system. The first three digits refer to alloy type,
the fifth and sixth digits represent 100 times the mean carbon
content. At the fourth digit letters, A, M and H indicate if the
steel is supplied to analysis, mechanical property or hardenability
requirements, which are the new alternative methods.

Ultra-high Tensile Structural
Steels

Interest in (1544-2160 MPa) tensile steels for use in
the aircraft industry is leading to the development of modified
steels which can be used up to 400°C in supersonic aircraft and
which possess adequate ductility and notch impact strength. Vacuum
arc or Electro-slag melting of some of these steels reduces
inclusions and impurities and gives a more suitable cast structure.
As a result, the transverse properties of the resultant forging tend
to be superior to those from conventionally air cast ingots. A few
typical compositions are given in Table 1. These are based on
Ni-Cr-Mo or use silicon with nickel or copper. Silicon reduces the
expansion in g-martensite change and hence reduces risk of quench
cracking; while copper is useful for producing secondary hardening
at 450-550°C.

TABLE 1. TYPICAL 0,37% C, 1850 MPa STEEL

Si

Mn

Ni

Cr

Mo

V

Temper

El

3% Cr-Mo-V

0,2

0,5

0,2

3,0

0,9

0,2

300

8

Rex 539 Si-Mn-Ni

1,6

1,6

1,8

0,1

0,4

0,2

350

7

H50

1,1

0,5

-

5,0

1,3

1,1

560

7

SAE 4340

0,3

0,6

1,8

0,8

0,25

-

220

9

Si-Cu steel

2,0

Cu=2

-

-

0,7

0,2

400

9

With such low alloy contents protection against rust by cadmium
plating or aluminium spraying is necessary. Unfortunately, cleaning
and plating processes can introduce hydrogen into the metal and
embattlement at the high strength level can be serious. Such
embattlement becomes evident under sustained loading, for example a
2,5 Ni Cr Mo steel hardened to 2220 MPa tensile broke within 100
hours at MPa and in 12 hours at 1544 MPa sustained load in air and
much shorter times in corrosive media. With such high tensile steels
it is highly desirable to (a) avoid notches, (b) to reduce internal
stresses by tempering at as high a temperature as possible, (c)
minimize the introduction of hydrogen (abrasive blasting preferred).
Cadmium coats should be avoided for service above 250°C. For other
high strength steels refer to precipitation hardening transformation
and controlled transformation stainless steels.

Maraging Steels

These use the martensitic reaction which end high hysteresis
occurs in Fe-Ni alloys. They are iron based alloys containing 18 Ni
8 Co 5 Mo with small amounts of Al and Ti and less than 0,03% C,
which makes such a difference to fracture toughness and ease of
welding. The strength is maintained with increase in section
thickness and also up to 350°C. Alloy cost is balanced by lower
production cost, virtually no risk of decarburisation, distortion or
cracking. These steels are used for air frame and engine components,
injection moulds and dies.

On cooling from the austenitic
condition the alloy transforms to a fine lath type martensite, and
precipitation hardening is induced by "maraging" at 480°C. Many
types of precipitates have been reported (e.g. Ni3TiAl) but the main
hardener is probably orthorhombic Ni, Mo, the solubility of which is
probably reduced by Cobalt. The steels have high fracture toughness,
KIc due to a combination of fine grain size of the martensite and
the high dislocation density, leading to fine precipitation. The
steels can be nitrided. The corrosion resistance is only slightly
improved but the 12% Cr variety has been developed for corrosion
resistance.

Hair-line Cracks

Many alloy-steel ingots and large forging are susceptible to the
formation of small silvery cracks or flakes in their interior. These
cracks often form at room temperature after an incubation period and
the cause of them is not completely known but is related to the
cracking of welded hightensile steels in that hydrogen has a large
influence in promoting embattlement which increases with the tensile
strength of the steel. Less trouble is experienced with acid
open-hearth steel (usually containing 4 cc per 100 gm) than with
basic electric steel containing about 6-8 cc per 100 gm of hydrogen
in the ladle. Slow cooling and also isothermal transformation at
about 600°C tends to reduce the incidence of hair-line cracks and
this is materially assisted by the vacuum melting. To reduce the
above hydrogen concentrations to about 1 cc per 1 00 gm requires
beat-treatments of the following magnitude at 650°C:

a) Cold drawn patented steel wire. b) Cold worked annealed
steel. c) Quenched and tempered (0,5/1,0%C) steel (VPN 340-430).
After having been formed, the springs, (a, b, c) are only given a
low temperature temper (170-300°C) to relieve forming stresses.
d) Annealed steel. After having been formed, the spring is then
quenched and tempered. All heavy springs are formed hot and
frequently hardened immediately after forming. Aero-engine
valve-spring steel must be free from any kind of surface defect.
Springs for watches and aircraft instruments, Bourden tubes,
diaphragms etc. are often made from Ni-span containing 42 Ni, 5 Cr,
2,3 Ti, 0,02 C, 0,55 Al which has low mechanical hysteresis and can
be heated to reduce the effect of changes in service temperatures.

Faults in springs are due to:

1) Decarburisation due to annealing, etc., affecting the fatigue
properties. 2) Segregations forming lines of weakness in the
material which may open up into splits in service. 3) Internal
cup and cone fractures due to overdrawing. 4) Mechanical damage,
such as rolling laps, deep grooves, scratches due to wire drawing,
vice marks and scoring due to winding. 5) Incorrect tempering,
especially in chromium-vanadium steels.

High and Low Thermal Expansion Steels

There are cases in engine construction where steel has to work in
conjunction with light alloys, such as cylinder-head bolts, valve
seating, or cylinder liners in aero engines. The comparatively high
thermal expansivity of aluminium leads to looseness unless the steel
has a similar coefficient of expansion. The austenitic steel of the
following composition

C, 0,59; Ni, 12; Mn, 5,1; Cr, 3,4

has a thermal expansion of 0,000021 per degree C up to 400°C,
which is only slightly lower than that of aluminium, and it combines
good mechanical properties with resistance to abrasion. Cold
rolled austenitic stainless steel is another alternative. Where an
abnormally low coefficient of expansion is required, Inver,
containing 36% Ni, is used.

Ball-race steel. A typical composition
is C, 1,0; Mn, 0,5; Cr, 1,36%. After quenching in oil from 810°C the
steel is usually tempered at 100-200°C to a) reduce hardening
stresses, b) reduce cracks in grinding.

Tempering at 100°C also increases the hardness slightly,
e.g.:

Tempering temperature nil 100 200 250 VPN 800 876 750 736

Creep-resisting Steels for Use at Steam
Temperatures

The use of higher temperatures and pressures in modern power
stations has necessitated the use of special steels for the
pipe-lines and other parts. The essential characteristic of these
steels is higher resistance to creep at temperatures varying from
400° to 565°C. A common steel used for this purpose is one
containing approximately 0,55% molybdenum with a carbon content of
approximately 0,15-0,2%.

Failures of pipes have occurred in America,
which have been traced to the formation of a network of graphite in
the heat-affected zone of the pipe adjacent to the weld. Heating the
steel to approximately 750°C appears to accelerate the formation of
this graphite, which is also largely affected by the process of
making a steel, particularly as regards the use of aluminium as a
deoxidizes, which is more commonly used in America than Great
Britain. It has been found that a small addition of a carbide
stabilizer, such as 1,0% chromium, is beneficial in minimizing this
trouble.

The addition of 25% V raises the creep resistance still
further. The 1% Cr Mo V steels currently in use are chiefly of two
types, those for steamchest castings where due to welding
considerations the carbon level is limited to 0,5%; and those used
in HP/IP rotors in which the need for improved hardenability with
large rotors has necessitated carbon levels of 0,25-0,30%.

Creep seems to be related to the uniformity of the V4C3 and its
interparticle spacing which should be less that 1000A (10-5 cm).
This distribution is affected by cementite which promotes regions
denuded of V4C3 and to minimize this problem the cooling rate from
the austenitising temperature must be such as to give an entirely
upper bainitic structure.

Low-carbon 3 and 6% chromium steels containing 0,5% molybdenum have carbides which resist hydrogen
attack and embattlement at elevated temperatures and pressures and
are useful in synthetic ammonia plants and oil refineries. Columbium
or titanium is sometimes added to minimize the air-hardening
tendencies in the steel by forming carbides less soluble at
heat-treating temperatures. Air-hardening still occurs after welding
but air-cooling from 800°C is sufficient to soften the steel.
Resistance to corrosion and oxidation increases with the chromium
content; the elongation at rupture is also increased.